Electrodeposition as a coating application method involves deposition of a film-forming composition onto a conductive substrate under the influence of an applied electrical potential. Electrodeposition has become increasingly important in the coatings industry because, by comparison with non-electrophoretic coating means, electrodeposition offers increased paint utilization, improved corrosion protection and low environmental contamination.
Initially, electrodeposition was conducted with the workpiece being coated serving as the anode. This was familiarly referred to as anionic electrodeposition. However, in 1972, cationic electrodeposition was introduced commercially. Since that time, cationic electrodeposition has steadily gained in popularity and today is by far the most prevalent method of electrodeposition. Throughout the world, more than 80 percent of all motor vehicles produced are given a primer coating by cationic electrodeposition.
Typically, electrodepositable coatings comprise an electrodepositable film-forming polymer and a curing agent, in combination with, inter alia, pigments. Lead-containing pigments such as lead silica chromate, basic lead silicate, lead chromate, and lead sulfate are often used in electrodepositable coatings because they impart excellent corrosion resistance to the electrocoated article. However, the acid used in cationic electrodeposition baths often solubilizes a portion of the lead pigment forming lead salts which are soluble in the aqueous phase of the electrodeposition bath. These lead salts often find their way into the ultrafiltrate of the bath, thus necessitating the removal and subsequent disposal of metallic lead and/or ionic or organic lead-containing materials.
In recent years, due to environmental concerns, particularly in Europe and Japan, the use of lead-free coatings has been mandated. Although surface coatings of excellent quality can be achieved by means of cationic electrodeposition of lead-free coatings, the removal of corrosion inhibitive lead pigments can result in reduced corrosion resistance of these coatings, particularly when applied to untreated or poorly pretreated steel substrates.
U.S. Pat. No. 4,789,441 discloses a metallic coating on a substrate applied by composite electrodeposition of a metallic matrix of nickel, cobalt or iron which contains particles of CrAlM.sub.2 where M.sub.2 is yttrium, silicon, or titanium. The composite electrodeposition metallic coating imparts corrosion resistance to substrates which are used in aggressive media and is particularly useful for coating gas turbine blades. This "coating" is completely metallic in nature and must be fused with the substrate at temperatures of greater than 700.degree. C., preferably over 1100.degree. C. in order to achieve diffusion of the deposited metals into the substrate. Such coatings are unsuitable for general use in common industrial painting applications.
The use of yttrium to improve corrosion resistance of conventional organic coatings is not known in the art. Nor is the effectiveness of yttrium as a corrosion inhibitor in conventional, cationic electrodepositable coatings known. It, therefore, would be advantageous to provide a lead-free electrodeposition bath containing a yttrium source which provides improved corrosion resistance of the electrocoated metal substrates, especially untreated steel.